1. Field of the Disclosure
Processes for modifying naturally occurring oils and fats with reactive silyl groups that cure simply upon exposure to moisture and related compositions are disclosed. Such compositions are produced by heating the oil and an unsaturated hydrolysable silane in the presence of a catalyst under inert atmosphere to graft the hydrolysable silyl group to the oil.
2. Brief Description of Related Technology
The use of renewable resources feedstock as an alternative to petroleum-based materials is desirable and advantageous as it will decrease the dependency on dwindling crude oil reserves and can reduce the danger of global warming. It is well known that natural oils and fats have been used as important components, especially as drying oils, in alkyd and similar resins. The cured coatings obtained from such drying oils have been noted for their anticorrosion properties, excellent water barrier, enhanced chemical protection, and high wear and UV resistant. These coatings are said to be durable under extreme weather conditions while also providing aesthetically pleasing luster finishes. The “drying” ability of these oil compositions is achieved by air oxidation when the unsaturation sites of the fatty acids undergo crosslinking upon exposure to oxygen. The presence of trace amounts of certain metal salts, such as cobalt naphtheneate, was usually added as a catalyst to attain sufficiently rapid cure times. Thus, such curable oils must include linseed oil or a drying oil having drying properties generally similar to linseed oil in sufficient amounts to yield said useful compositions. However, linseed oil and similar drying oils, which are derived from multitude of unsaturated fatty esters in their triglyceride structure are relatively expensive and are not commonly available. Further, the oxidation reaction that leads to drying and curing of these oils is highly exothermic and care must be exercised to prevent fire hazards when working with these oils. In practice, many compositions that include linseed oil and the like must be diluted in a volatile organic solvent, which would evaporate into the atmosphere upon application and would contribute to air pollution. Alternatively, the oil was dispersed in water, instead of dissolving it in organic solvents, however, such suspensions required the use of surfactant stabilizing agents, which remained in the finished coating and degraded from its physical properties and its aesthetic appearance as described in details in T. Nabuurs et al., Prog. in Org. Coating 27, 163 (1996). It is therefore desirable to have curable compositions derived from common and readily available fats and natural oils that will cure in a single stage upon exposing them to the atmosphere without all the deficiencies of the air oxidation oils.
Much effort has been directed to modify the structure of common fats and oils to yield specific reactive sites that could be crosslinked and therefore function as a drying oil. Examples include formation of epoxides, hydroxyls, aldehydes and grafting of polymers as reviewed by Biermann et al. in Angew. Chem. Int. Ed. 39, 2206 (2000). However, this approach requires either activation of the cure system by applying heat or initiating the cure by mixing the oil with an initiator that is stored in a separate compartment from the oil.
There are many patents related to silicone sealants that are based upon the use of alkoxy functional polysiloxanes and alkoxy functional crosslinkers. Representative of the prior art is U.S. Pat. No. 3,334,067 to Weyenberg. This patent discloses a method of making one component room temperature siloxane rubber whereby the compositions are stable in the absence of moisture, but cure upon exposure to moisture. The method comprises mixing in the absence of moisture a hydroxyl end-blocked siloxane polymer with an alkoxy silane of the formula R′Si(OR″)3. Similarly, U.S. Pat. No. 3,383,355 to Cooper discloses polysiloxanes having alkoxy groups bonded to terminal silicon atoms. These functional diorganopolysiloxanes having from two to three alkoxy groups attached to each terminal silicon atom are curable in the presence of moisture and a suitable hydrolysis and condensation catalyst to yield a rubbery material. U.S. Pat. No. 3,856,839 to Smith et al. also discloses the cure of a composition containing methyltrimethoxysilane and a silanol terminated polydiorganosiloxane fluid. However, none of the teachings related to the incorporation of the reactive silanes to polyorganosilicon to produce compositions that will cure by moisture can be used to attach the reactive silanes to natural fat and oils. This is the case since it is not possible to attach the moisture sensitive reactive silanes by condensation or hydrosilylation to natural oils and fats as has been taught in the prior art.
There are only a few examples where reactive silanes were attached to natural oils and fats; U.S. Pat. No. 3,424,598 to Snyder et al. discloses the reaction of trichlorosilane with beef tallow, soybean oils or fatty substances esters of trihydric alcohol containing unsaturated acyl groups to prepare water repellent surface coatings for concrete. Accordingly, the trichlorosilane was initially reacted with the oil under nitrogen bubbling in ether and then irradiated for 48 hours at 35° C. with Hg lamps. Upon removing the solvent and excess chlorosilane, a syrupy colorless liquid was obtained. It is claimed that such products when applied to cured concrete reduced the water absorbance of the concrete, provided good weather resistance (including freezing and thawing) and showed excellent resistance to microbial degradation. It is apparent to these expert in the art that this method of UV catalyzed hydrosilylation is limited only to chlorosilanes and will not proceed to any significance with other silanes if the double bonds are not in the terminal position. Furthermore, the use of chlorosilanes is not desirable since the by-product of the moisture activated cure is strongly acidic HCl, which will cause saponification of the triglycerides. The presence of HCl is further undesirable as it can lead to corrosion of any metallic substrates or substrates that contain metals (as in reinforced concrete).
U.S. Pat. No. 2,735,825 to Kress discloses silicone-modified alkyd coating having superior weather and chemical resistance and good curability that were obtained by reacting oil-modified alkyds with acid hydrolysis products of chlorosilanes and then diluted in a solvent. Similarly, U.S. Pat. No. 2,717,900 to Plueddemann and BE Pat. No. 510,419 issued Aug. 1, 1952 to Goneberg disclose silicones containing oils that were prepared by the reaction of polyhydric alcohols with mono- or diglycerides of fatty acids and a silanol. Such compositions are derived by attaching the silicon group via a Si—O—C linkage. However, these compositions and similar examples in the prior art of mixtures of various silanes and natural oils, although said to be “curing”, are not capable of crosslinking via hydrolytically stable covalent bonds to a network and the so called “cure” is not activated by atmospheric moisture.
Another approach is disclosed in U.S. Pat. No. 4,512,926 to Kampf which discloses a process for the silylation of unsaturated, naturally occurring oils by reacting them with appropriate silanes at 200° C.-350° C. under an inert gas and in the absence of a catalyst/initiator. The products of this invention are claimed to be useful for soil stabilization, or additives to improve the properties of adhesives, cements, sealing and caulking compounds or as dispersion agents for pigments. Additionally, they can be used to render substrates, such as paper, textile materials, wood, cardboard, and building materials, hydrophobic. The teaching of Kampf, however, produces poor network gels where a substantial amount of the oil molecules remains unreacted and uncurable. Thus, upon exposure to moisture, substantial fraction of the composition remains liquid that tends to migrate out of the bulk and to stain the substrate surface and the surrounding areas.
Objects
A more convenient approach would be a single compartment system where all the components are stored together and the cure is activated by exposing the formulation to atmospheric moisture at ambient conditions such as in compositions containing reactive silicones. The moisture-activated cure chemistry is based on hydrolysis of organic groups attached to a silicon atom through hydrolytically unstable covalent bonds such as in alkoxy silanes. Upon hydrolysis of the alkoxy groups by atmospheric moisture, silanol groups are formed, which are then condensed to yield stable crosslinked siloxane bonds. Further, it would be desirable to provide a curable oil that has a relatively low viscosity (i.e., so that it can be conveniently applied to a substrate) and that exhibits limited or no leaching upon cure.
These and other objects may become increasing apparent by reference to the following description and the drawings.